This invention relates generally to methods and apparatus for controlling a device with a control signal responsive to a feedback signal. In a particular aspect, the invention relates more particularly to a method and apparatus for controlling a screw conveyor that moves proppant into a tub for mixing into a fracturing fluid used to fracture an oil or gas well.
Various devices are controlled in closed loop control systems wherein a controller (1) generates a control signal for trying to cause the device to operate so that a previously defined set point condition is reached and (2) receives a feedback signal in response to which the controller varies the control signal as needed until the set point condition is met or changed. Such a control system may work well until the controller fails to receive the feedback signal, at which time the controller does not know what the controlled device is doing in response to the generated control signal.
One system we are aware of does not recognize a no feedback signal condition as such. Instead it assumes a zero feedback signal, which causes the controller to generate a maximum control signal. This can result in the device being overdriven into a runaway condition. When the system is trying to control amounts of materials being mixed together, for example, such a condition can result in a mixture having the wrong amounts of constituents.
To overcome this shortcoming, there is the need for a method and apparatus for controlling a device with a control signal responsive to a feedback signal so that intelligent responses can be automatically provided when a feedback signal is lost. Such a method and apparatus should be able to determine whether manual control of the apparatus should be implemented or whether continued, but modified, automatic control should be maintained. If the latter, the method and apparatus should also be capable of determining how to make the necessary adjustment automatically.